The Internet Protocol (IP) Multimedia Subsystem (IMS) architecture is evolving as the service infrastructure for IP networks. The IMS architecture supports services such as voice-over-IP (VoIP), multimedia sessions, presence, instant messaging, gaming, and various other services. The Session Initiation Protocol (SIP) is an important part of the IMS architecture. In support of such services, SIP messages are routed between SIP network elements (including SIP end user equipment) according to the SIP protocol as defined in RFC3261, RFC3263, and associated extension documents. In a SIP network, equipment that initiates a SIP message is referred to as a SIP client (e.g., SIP User Agent Client) and the equipment that terminates a SIP message is referred to as a SIP server (e.g., SIP User Agent Server).
The SIP messages transmitted between SIP clients and SIP servers traverse SIP network elements supporting various functions. For example, SIP message transmitted between SIP UACs and SIP UASs may traverse SIP proxy servers that determine the next SIP element in the routing path, SIP registrars that register SIP device locations in databases, SIP application servers that perform application specific operations based on SIP message content, and various other SIP network elements. Since the IMS architecture supports a variety of different services, SIP messages may differ in their importance or urgency (e.g., the set of SIP messages required for establishing an E-911 emergency call may have a higher importance and urgency than respective sets of SIP messages required for establishing standard VoIP calls).
In general, the number of SIP messages in a SIP network element represents the load of the SIP network element. A high-load condition occurs if a SIP network element receives more messages than it is capable of processing. Several measures are available to deal with high-load conditions. For example, SIP network element hardware equipment may be upgraded, or load distribution strategies may be applied in order to divide SIP messages across several SIP network elements. Disadvantageously, however, despite careful network engineering, SIP network elements may experience high-load and overload conditions, thereby resulting in SIP message delays and drops independent of the importance or urgency of the SIP messages.